Hmm.. well Dreamcast is very limited as a MAME platform, the development of the two preeminent DC MAME projects (MAME DC and MAMED DC) have stalled significantly, probably due to the Dreamcast going EOL. So I would write the DC off as irrelevant (he says, preparing to toss his DC out the window.)

I'm not sure which would cost more but then you've got the controls issue... That seems to be a larger stumbling block than the monitor. XBox doesn't have arcade-quality controls. I am sure people will have kits and adapters to adapt arcade-style buttons to XBox inputs, but I haven't seen any yet.